Retinal and neural membranes contain high levels of docosahexaenoic acid (22:6w3, DHA), an omega-3 fatty acid. We have shown in infant rhesus monkeys that dietary deprivation of omega-3 fatty acids during gestation and postnatal development leads to reduced brain and retinal DHA levels and is associated with deficits in retinal function, visual acuity and visual attention. These findings led to increases in the amounts of omega-3 fatty acids in many infant formulas, and prompted studies of premature human infants which found similar effects of omega-3 fatty acid nutrition on all of these measures. We are now investigating the optimal form of omega-3 fatty acids in infant nutrition linolenic acid (LnA), DHA's precursor, which is the form provided in all current U.S. formulas, or preformed DHA, which is present in human milk. We are midway through a five-year study comparing rhesus infants fed formulas containing low LnA, high LnA, or preformed DHA plus other essential fatty acids found in human milk. Erythrocyte DHA levels of infants in the low LnA group were lower than in the high LnA group, but both were 70-90% below those in the DHA group, which matched the values for breast-fed rhesus infants. We are evaluating several aspects of visual and cognitive development in these infants. Through 4 months of age, retinal function was equivalent in infants fed the high LnA and DHA diets. Measures of visual attention confirmed differences between the low LnA and high LnA group, but the DHA group's performance was intermediate. Thus, preformed dietary DHA appeared to confer no advantage for retinal or visual system development. Behavioral and cognitive evaluation of these groups is continuing. The results will help to define the nutritional importance of omega-3 fatty acids and to guide the formulation of future products for human infant feeding.